The present disclosure relates to a separator, battery using the same, and a method for manufacturing the separator.
With the remarkable development of the portable electronics in recent years, cellular phones and notebook computers have been recognized as infrastructure technologies for an advanced information society. The research and development related to the high-performance of these devices have been intensively proceeded. As power consumption increases with the high-performance, there is a need for shortening the drive time. In order to ensure the drive time above a certain level, an essential condition is to increase an energy density of a secondary battery to be used as a driving source. Thus, for example, a lithium-ion secondary battery is expected.
In the lithium-ion secondary battery, the internal energy is large and thus a high safety is necessary during abnormal conditions such as an internal short circuit and an external short circuit. For safety measures, a polyolefin separator produced by drawing polyolefin has been used. In order to ensure the safety, the polyolefin separator has a shutdown function to make electric currents interrupted in the case where the battery abnormally generates heat. The shutdown function is a function in which the separator is fused at a temperature near the melting point and a hole area is sealed, thereby interrupting electric currents.
However, the shrinkage of the separator occurs when it is shut down. Thus, a secondary defect such as an internal short-circuit may be caused by the contact of a cathode and an anode. It is necessary that the heat shrinkage is reduced by improving the heat resistance of the separator in order to improve the safety. For example, a separator in which an inorganic oxide porous membrane is formed on an organic porous membrane in order to improve the safety is described in Japanese Patent Application Laid-Open (JP-A) No. 2004-14127.
On the other hand, although it is not related to the structure of the separator, the production method of SiO2 based ceramic coating film which contains polysilazane as a raw material is described in Japanese Patent No. 3771314.
Considering of high capacity and high safety of the lithium-ion secondary battery, a sufficient capacity cannot be obtained by using only polyolefin resin fine porous membrane in related art. Therefore, it is necessary to give the function to the polyolefin resin fine porous membrane. As a method for giving the function to the polyolefin resin fine porous membrane, for example, a method for coating resins having different properties on the polyolefin resin fine porous membrane has been proposed. For example, a separator in which resins such as polyvinylidene fluoride and polypropylene is coated on the polyolefin resin fine porous membrane in order to improve the electrochemical stability of the separator is disclosed in JP-A No. 2006-286531.
It is desirable to provided a separator capable of reducing the heat shrinkage and improving safety, battery using the same, and a method for manufacturing the separator.
In the separator described in JP-A No. 2006-286531, the electrochemical stability of the separator can be improved. However, a resin layer formed by coating is inferior in the impregnation property of the electrolytic solution and the ionic conductivity. Therefore, satisfactory battery characteristics could not be obtained.
Therefore, it is also desirable to provided a separator which is excellent in impregnation property of the electrolytic solution and to which the resin function is imparted without impairing the ionic conductivity and a battery using the same.